Conventionally, a polyurethane foam which is foamed using HGFC such as Freon was mainly used for an insulator, however, a new heat insulating system is required recently because a heat insulating function is reduced by deterioration when it is used for a long time and using of the HCFC is restricted because of environmental problems. In addition, a vacuum heat insulating system is searched in order to improve insulating characteristics of products such as a refrigerator or a freezer.
The vacuum insulating system is a system for minimizing heat transfer and maintaining heat insulation by maintaining inside the system to be vacuum. In order to maintain the vacuum as a panel state, important elements are strength and air pore state of a main body, and heat insulating function by which the atmosphere pressure can be endured.
The main body used in the vacuum heat insulating system can be made using inorganic materials such as silica powder, fiberglass, molded body of calcium silicate, and using organic materials such as polyurethane and polystyrene foam of fine open cell structure.
Methods of using the silica powder as the main body of the vacuum insulator are disclosed in U.S. Pat. Nos. 4,195,395, 4,425,413, and 4,681,788, a method using the polyurethane foam of open cell structure is disclosed in U.S. Pat. No. 4,668,555, and a method of using the polystyrene foam is disclosed in Korean Patent Application No. 95-48619. In addition, there is disclosed a method of increasing insulation function by changing the air pore structure of the polystyrene and polyurethane foam into an anisotropic structure in Korean Patent Laid-open No.1999-010954. The heat insulating performance of the vacuum insulator made by the insulating main body like above is 0.004˜0.007 Kcal/mh° C. which is better than 0.02 Kcal/mh° C. of the polyurethane foam in conventional refrigerator, however, it has lower performance than 0.002˜0.003 Kcal/mh° C. of the vacuum insulator made using the fiberglass.
The vacuum insulator using the fiberglass is disclosed in U.S. Pat. Nos. 5,090,981, 5,094,899, and 5,330,816.
According to the above US patents, the fiberglass should be molded as a certain form in order to be used as the vacuum insulator, and therefore methods of using small amount of organic or inorganic binder, or compression heat hardening the fiberglass not including the binder are used. In case of using organic or inorganic binder, the binder may occupy the air pore layer of the fiberglass, or the deterioration is generated, whereby the vacuum insulating effect is reduced and producing cost can be increased because of increasing processes and materials. According to the method of heat hardening, the fiberglass is compressed in a temperature range of higher than strain point and lower than softening point of the glass constituting the fiber glass, and equilibrium time is maintained, whereby a fiberglass board is made. However, according to this method, the fiberglass should be heated to the temperature higher than the strain point of the glass constituting the fiberglass, and sealing should be made in a vacuum chamber when vacuum air evacuating, and therefore many processes and energy are required and producing cost is increased.